The invention relates to the use of a papermachine fabric for the production of tissue paper or a porous batt. The papermachine fabric consists of two interconnected fabric layers with the lower fabric layer being coarser, i.e., having lesser density of longitudinal and transverse wires with said longitudinal and transverse wires having a larger diameter than those in the upper fabric layer. The invention further relates to papermachine fabrics which are especially suited for the manufacture of tissue paper or a porous batt.
Conventional two layer papermachine fabrics of the type described in EP-A-No. 0,044,053, DE-A-No. 2,455,184 and U.S. Pat. Nos. 2,455,185; 2,917,694; 3,305,713; and 3,329,740, are used for the manufacture of paper, e.g. newsprint, and are not suited for the manufacture of tissue paper or a porous batt where structuring by different fiber density or pattern-like fiber concentration is desirable.
It has been known in the manufacture of porous tissue paper to provide sheet forming fabrics with projecting impermeable synthetic resin areas distributed in the manner of a pattern on which no sheet forming takes place and therefore holes are left in the paper sheet (DE-A-No. 1,786,414). Furthermore, it has been known to form thin areas in the paper web during sheet forming on a very coarse fabric by projecting warp knuckles (U.S. Pat. No. 1,102,246).
It is also possible to emboss a soft, bulky tissue paper web by a so called embossing fabric in such a way that compacted areas alternate with soft material in the paper (U.S. Pat. Nos. 3,301,746; 3,629,056; 3,905,863; and 4,440,597, and DE-A-Nos. 2,820,499; and 3,008,344). The moist paper web in this process is supported by a coarse fabric. When hot air is forced through fabric, the paper web assumes the configuration of the supporting fabric area and the hot air stream forces the batt into the depressions between the projecting warp knuckles. In all these cases single layer fabrics are used and the embossed pattern depends on the fabric weave. The height of the projecting embossing knuckles is predetermined by the fabric structure which, in turn, is variable to only a limited extent. In order to make the embossed areas more pronounced, the surface of the projecting warp knuckles is abraded.
Recently a method has become known (EP-A-Nos. 0,135,231, and 0,140,404) in which the paper web is embossed with a honeycomb-like pattern. After the paper web has been formed on the sheet forming fabric the moist web is taken over by the embossing fabric and deformed in the desired way. Said embossing fabric consists of very fine fabric with 17 longitudinal and 18 transverse wires, each having a 1.18 mm diameter. The open area amounts to forty-five percent. A hexagonal honeycomb structure of photosensitive resin is applied to the fabric. The paper web is drawn into the depressions of the embossing fabric by the action of a suction box whereby the fiber structure of the paper web is changed. The paper web is after dried on the embossing fabric from about ten percent to sixty-five percent, first by the action of the suction box and thereafter by blowing hot air therethrough. The paper web is then pressed onto a heating cylinder by a pressure roll. This pressing treatment intensifies the embossed honeycomb structure because the embossing fabric travels between the pressing roll and the paper web. In order to increase the adhesion of the paper web to the drying cylinder an auxiliary adhesive is sprayed onto the web and onto the cylinder.
The paper produced with this method meets the product requirements by the method has the disadvantage that the embossing fabric is very weak and unstable. The supporting fabric must be very open and has but low stability in the longitudinal and transverse directions, a fact which promotes the formation of ridges and folds. Furthermore, it is extremely complicated and expensive to produce the honeycomb structure. Also, the photosensitive resin causes high abrasion at the suction box which is a drawback where high friction soon wears down the very fine fabric on the running side. A major problem is soiling of the fabric by the auxiliary adhesive employed by which the paper web is adhered to the heating cylinder. The fabric must be continuously cleaned with a highly pressurized water jet. Although the adhesive is rinsed off, the webs of the honeycomb pattern may break off and after a short time the embossing wire becomes useless.